The US is to shut its monitoring unit in the UK, marking the end of nearly 75 years of collaboration with the BBC's open-source intelligence division at Caversham Park. The Open Source Enterprise, a division of the Central Intelligence Agency, has been run out of the Berkshire stately home since 1943. US officials have worked closely with their British counterparts to monitor foreign television and radio broadcasts, as well as online information.

However, with the BBC committed to selling what was once known as Britain's listening post and moving its operations to central London next year, the future of the American service has been cast into uncertainty. Despite discussing a possible move to RAF Wyton, an air base near Cambridge, the US government has now said that the OSE is to close the bureau altogether, confirming a story first reported by Private Eye magazine.

"The Open Source Enterprise will be shutting down its UK-based bureau with the closure of Caversham Park, but it remains fully committed to sustaining and growing its relationship with the UK government,"

said the US embassy in London.

The embassy did not disclose how it would continue to gather the intelligence or whether the unit would move back to the US. One senior executive with knowledge of the situation said one option being considered was to outsource the work to a private contractor. Under sharing arrangements, the US covers 75 per cent of the world, while the UK covers the remaining 25 per cent.

The decision to wind down OSE is likely to provoke criticism from the British politicians who have strongly opposed the decision taken by former prime minister David Cameron's coalition government in 2010 to scrap ring-fenced government funding for BBC Monitoring.

"This means the fulfilment of our worst predictions,"

said Julian Lewis, the Conservative MP who chairs the UK Defence Committee.

"Anyone who thinks that breaking up this dedicated centre and reassembling the pieces on separate continents will be anything like as effective and sophisticated is living in an unreal world."

Confirmation of the US plans to shut the unit comes a week after Boris Johnson, foreign secretary, said he planned to visit Caversham Park.

Although the BBC and the US government played down the possible impact on the sharing of information between the two countries, the move may raise new questions about the strength of the so-called special relationship. On Thursday, John Sawers, the former head of the Secret Intelligence Service, MI6, warned that Brexit threatened Britain's influence on the world stage just as Donald Trump's US was becoming "less interested in working with partners".

"Our foreign policy, our defence policy has been predicated for the last 75 years on a very close partnership with the United States,"

Sir John said.

"We are entering into an era where our ability to rely on that partnership is not as great as it was in the past."